The use of crystal devices to generate reference signals is well known, and well established in the art. They are available in a wide range of frequencies, and the techniques for using them and associated circuitry are likewise well known. It is also known that a crystal's electrical characteristics are temperature dependent, meaning that as the temperature of the crystal changes, so does its frequency. Generally this dependency is non-linear. In many applications the change of frequency over temperature is of little consequence. In other applications, however, it is critical to have a stable reference frequency. One area in which a stable reference frequency is critical is in radio communications, where frequency error resulting from temperature shift could put the radio device on an incorrect frequency channel, or cause numerous other problems.
There are two main ways of dealing with the effects of temperature in crystal based oscillators. The first method is to measure the specific temperature characteristics of each crystal, such as its Beckman's curve, and a store the information in the device in which the particular crystal is to be used in, so that the device can control the oscillator circuit accordingly and compensate for temperature effects. The second method again involves measuring temperature dependency information, but storing the information in a memory device integrally housed with the particular crystal element.
Perhaps the best example of the first method is the where a crystal manufacturer measures the temperature information at the time of manufacture, and prints the encoded information on the case of the crystal reference so that manufacturers who use the crystal can scan the crystal with an optical scanner, obtain the information, and program them into the device in which the crystal is being assembled. This is straightforward enough, but has the obvious problem in manufacturing of making sure the scanned part is actually placed into the corresponding device. It also make field replacement difficult because field offices must have the right scanning equipment, and be able to reprogram the device with new crystal information. Finally, it is preferable in sensitive electronic devices using crystals to place a faraday shield over the crystal to prevent interference. However, since the temperature information is printed on the crystal, a shield can not be placed until after the information has been read, complicating assembly operations.
The second method is in limited use because, by including a memory means with the crystal, the cost of producing the crystal reference increases. In addition, implementations of this method increase the number of electrical connections to the crystal reference to access the memory device. Additional electric terminals generally translate into more opportunities for failure, more complex circuit layout, and increased cost to manufacture the crystal. Therefore, there is a need for a crystal reference which overcomes these deficiencies associated with the prior art.